“Fear is a wonderful thing, in small doses,” Neil Gaiman once said. “You ride the ghost train into the darkness, knowing that eventually the doors will open and you will step out into the daylight once again.”

These days it feels like there are fewer and fewer ways to get on that “ghost train” of delighting in the suspense of a scary story. The tradition of swapping tales around a campfire is a rare occurrence in modern life, and smartphone screens too often get in the way, and stop us from getting lost in a book.

But a community of horror writers and fans who populate the NoSleep subreddit seem to have found a way to keep the “ghost train” alive. NoSleep has more than four million subscribers; popular posts there can garner tens of thousands of readers in a single day. The audience is mostly American, but it also has substantial followings in Europe and, improbably, Polynesia.

“Everything is true here, even if it’s not,” the sidebar on the website reads. And this rule of etiquette is key to maintaining the experience of the story. As a moderator of the subreddit, who goes by the name Kerrima, told the Guardian, her favourites are “creepy, sort of subtle stories, where maybe something happened, maybe something didn’t happen”.

The vague, unresolved nature of the stories are key to their appeal. The best ones are rarely gory, a function of the fact that images are more or less banned – you can link to them, but only sparingly. The scares come by way of the unknown.

Take, for example, the stories NoSleep’s moderators cite as their favourites. Kerrima likes a story called Free Coffee with Order of Pie, which deposits the reader in a diner in the middle of nowhere that is visited by a mysterious, prescient stranger.

Meanwhile, Khristopher Patten, another moderator who by day is a PhD student in cognitive neuroscience based in Arizona, said his favourite is a series called Correspondences, written by a user who goes only by the name “bloodstains”. The story, which you could say is about demonic possession, is told in an epistolary mode, each episode proceeding by emails, texts and blog comments.

What both stories have in common is that it’s impossible to know exactly what happened; the unsettled feeling they leave behind is wholly a function of the answers the authors never give.

What many NoSleep stories have in common is that it's impossible to know exactly what happened

But the indeterminacy of a NoSleep story can backfire. For example, just under a year ago a popular NoSleep writer posted an item titled WTF is going on in Pinal County, Arizona?. It described the outbreak of a mysterious disease in a small town called Mammoth, population 12,000. The epidemic began at a daycare, the writer said. The elderly owner had collapsed and died after bleeding from the ears and screaming in a fit of rage – and the children had been with her at the time.

The plague had then spread to the kids, and to their families. The mysterious disease was now afflicting the poster’s sister. The CDC, the writer said, was ignoring her frantic calls, but some kind of authority seemed to be closing in. The post cut off with the writer pleading: “The sirens have me terrified and the sun is almost down here.”

Pretty standard stuff for a horror story, not even particularly original. But apparently a few casual readers, coming across the story (which was actually part of a series, but only the Arizona instalment took off), felt concerned. They called the Mammoth police department, and they called local businesses to ask if the online reports of the epidemic were true. At that point, the media got involved.

Both the Arizona Republic and USA Today wrote stories about the incident. They likened it to the famous hoax of the 1938 radio broadcast of HG Wells’ The War of the Worlds, long said in urban legend to have terrified a large swath of America. Yet just as some historians contest that any “mass panic” followed that broadcast at all, it’s not clear Mammoth residents were overly bothered by the confusion. In fact, an audio recording of a call to a local store in Mammoth that one NoSleep reader made at the time, sees a clerk giggling about the fuss ginned up by a “damn website”.

For their part, NoSleep’s regular users were sharp on a separate thread about the “overly paranoid silly geese” who made the calls, writing that they deserved about “three years of time out”. To them, calling the police about a story posted on NoSleep is the decided mark of a rube.

Perhaps cynicism is an obvious byproduct of spending so much time in a state of suspense.

In fact, in an interview this week, NoSleep’s moderators seemed more irritated about the journalists who sometimes drive by and lift stories onto their own websites, misleading casual readers to believe they are true. Both BuzzFeed and Jezebel, for example, republished a NoSleep story about a boyfriend who was receiving Facebook messages from his dead girlfriend. To a regular reader of NoSleep the thing was plainly fiction, but several websites used a headline that made it sound like fact: “This Guy’s Story About His Dead Girlfriend Facebooking Him Might Be The Scariest Thing On The Internet.”

“There was someone who tried to write a story that solved a real crime,” said Kerrima. “Like, it’s a fictional solution to a real crime that happened.” The crime involved the disappearance of a woman, and someone who had known the victim wanted it removed. “I just didn’t want the hassle of having abuse hurled at the sub,” Kerrima said, so she removed it. The moderators also deleted a couple of speculative stories posted about the fate of the doomed MH370 plane, on grounds that they were posted too close to the real events.

There are, as it turns out, a surprisingly sophisticated set of moderating principles that go into maintaining the immersive fictional powers of NoSleep. The moderators mostly select and discard stories along a standard they call “believability”. In their posting guidelines, they define the standard thus:

As a general rule, if a reader could look outside and disprove your story, it’s not going to work for nosleep. If your story is physically impossible to post (like the narrator dies before the end, or it was a cat/infant the whole time), it will be removed. We don’t like to explicitly ban certain content, but things that “break the world” are never appropriate for nosleep. Examples include apocalyptic scenarios, large-scale time travel or alternate universes, or mass extinctions. Stories posted in second person narrative are not considered believable, and will be removed.

There’s a lot left up to discretion and interpretation in that standard, and the rules have shifted over time. For example, in another place on the site they say they will ban stories about zombies. But when I ask about it, I learn the rule has loosened.

“It used to be that the guidelines were a lot more strict,” Khristopher Patten, a doctoral student in cognitive neuroscience who is another of the site’s moderators.

“We kind of discourage putting labels on the monsters in a way, so if someone says, ‘I saw a vampire!’ that’s not okay,” Kerrima said, “But if someone said, ‘I saw a guy with pointy teeth,’ that’s OK.”

The specificity of the rules is no doubt the function of pure obsession. But they are also smart, a kind of roadmap for successfully scaring people. And they do so in a world whose frightening elements often feel like folklore than the ghosts and witches that bothered our ancestors.